This invention relates generally to thermal transfer printing, and more specifically, the present invention is directed to certain ink donor films useful, for example, in thermal transfer printing processes. In one specific embodiment of the present invention there is provided an ink donor film comprised of, for example, a supporting substrate, and thereover an ink containing a blend of waxes, pigment, or dye particles and certain polymers. Moreover, in accordance with another embodiment of the present invention there is envisioned a single strike ink donor film comprised of a supporting substrate, such as Mylar, and an ink containing a blend of four waxes, preferably hydrocarbon waxes, polymers such as poly(vinylmethyl ether), pigment particles and a pigment dispersant. Moreover, in accordance with another embodiment of the present invention there can be selected a multistrike ink donor film comprised of a blend of four waxes, certain polymers such as styrene-b-isoprene-b-dimethylsiloxane triblock copolymer, and oil soluble dyes or pigment particles with a dispersant. With the aforementioned ink donor films there results, subsequent to transfer, images of excellent resolution with no background deposits thereon, and wherein the ink selected exhibits low viscosity, acceptable thermal stability, and superior adhesion to the donor substrate.
In thermal transfer printing, there is selected an ink donor film consisting of a thin substrate coated with a waxy ink. In operation, the inked portion of the donor film is placed in contact with a plain paper receiver sheet, a smooth calendered paper, or a Mylar sheet followed by directing heat from a thermal printhead to the back of the ink donor film substrate causing ink to melt and wet the receiver sheet. Subsequent separation of the sheets provides for the transfer of ink to the image areas of the plain paper. Various substrates are of interest for the ink donor film including highly calendered condenser papers with thicknesses of from about 10 to about 20 microns and polyester (Mylar) films. Thermal printers utilizing such materials are selected for a variety of applications including facsimile, printer plotters, and computer output printers. Also, as office automation increases, low cost thermal printers appear to be prime candidates for communicating terminals allowing for office quality hard copy inputs. The main advantages of these printers are reliability, compactness, speed and low cost.
Generally, thermal printers can be classified as direct and transfer. In the direct system, a paper having a thermally sensitive coating, either a wax or an organic metal compound, is selectively heated causing color changes in the coating. In the transfer type of printer, a donor ribbon loaded with a marking material, typically a heat sensitive ink, is transported to a thermal printhead and a plain paper recording sheet. The printhead is electrically activated to selectively apply heat to the donor sheet thereby causing the melting and the transfer of a portion of the marking material onto paper in image configuration.
Disclosed in U.S. Pat. No. 4,273,602 are heat sensitive recording materials comprised of a support sheet of a thickness of from 5 to 40 microns having thereon a heat sensitive transfer layer with a phenolic material, a colorless or precolored material which reacts with the phenolic material to form color upon application of heat, and a heat fusible material having a melting point of 40.degree. C. to 150.degree. C. with an image receiving sheet superimposed on the surface of this layer. It is indicated in this patent that heat sensitive transfer layers can be formed from waxes or resins of low molecular weight with colored dyes dispersed therein, however, apparently there are problems associated with such a method in that part of the layer transfers to ordinary paper causing undesirable staining and a decrease in contrast between letters and the background. Accordingly, the recorded letters cannot be easily read.
Also, there is disclosed in U.S. Pat. No. 4,476,179 an ink donor sheet comprising a porous sheet and a heat-fusible ink layer formed thereover. The ink layer selected has a temperature-dependent gradation characteristic that enables it to remain solid at ordinary temperatures, and further when it is heated to a certain temperature its viscosity is suddenly decreased, and upon further heating, the viscosity is decreased gradually. The aforementioned ink donor sheet is capable of producing halftones on the images it creates.
There are described in application U.S. Ser. No. 666,702 relating to thermal transfer printing sheets, the disclosure of which is totally incorporated herein by reference, thermal transfer imaging sheets containing certain coating compositions thereof. More specifically, there are illustrated in this application transparencies suitable for the effective transfer of developed images from a donor sheet comprised of a transparent substrate, and a coating thereto selected from poly(vinylethers), poly(acrylic acid esters), and poly(methacrylic acid esters).
Additionally, the preparation of transparencies by electrostatic means is known. There is thus disclosed, for example, in U.S. Pat. No. 4,370,379 a method for preparing an original for projection according to electrophotographic processes. More specifically, it is indicated in this patent that the conventional method for preparing a projection original for an overhead projector, (a transparent sheet) according to electrostatic photography, comprises transferring a toner image formed on a photosensitive plate onto a stretched polyester film, and fixing the transferred toner image on the film by heat. Various plastic films can be used for this purpose including a biaxially stretched polyester film. It is further indicated in this patent that the transfer film selected for electrostatic photography is comprised of a film substrate which is transparent, such as a biaxially stretched polyethylene terephthalate film including Mylar films.
Moreover, there is disclosed in U.S. Pat. No. 4,234,644 a composite lamination film for electrophoretic toner images deposited on a plastic dielectric receptor sheet comprising in combination an optically transparent flexible support layer, an optically transparent flexible intermediate layer of a heat softenable film applied to one side of the support layer, and wherein the intermediate layer possesses good adhesion to the support layer. It is indicated in this patent that the support layer can be prepared from various suitable substances including polycarbonates, polysulfones, polyethylene terephthalates, Mylars, and the like.
In addition, it is known that thermal transfer printing utilizes single strike ink donor films, however, about 90 percent of the film is discarded rendering the selected process uneconomical. To alleviate this disadvantage, efforts have been devoted to the development of multistrike ink donor films wherein the film is reusable, that is for example wherein the ink donor film releases a controlled amount of ink which is transferred onto a receiver sheet. This objective is achievable with the invention of the present application. Accordingly, there is a need for ink donor films with waxes thereon that are substantially permanently adhered to the film component. Moreover, there is a need for ink donor films useful for the transfer of images to appropriate substrates, and wherein the images are of excellent resolution and retain their image density for extended time periods. Furthermore, there is a need for economical ink donor films useful in thermal transfer printing processes, and wherein the films comprise specific copolymers containing therein waxes and pigment components. There is a further need for multistrike processes wherein the ink donor film selected can be reused for in excess of about 7 image formulations and transfer processes. Also, there remains a need for ink donor films wherein acceptable images on transparencies are achievable. Furthermore, there continues to be a need for ink formulations present in ink donor films, which films are useful as single strike or multistrike components in, for example, thermal transfer printing apparatuses.